What I'm doing is trying to adapt a hardpoint-based system in a way that allows you some 3025-appropriate retooling and repair of your Mechs, but in a way that preserves the essential character of each Mech variant. One of the particular effects of this design has been the realization that I basically can't let you swap anything else into a MG slot, regardless of what 'hardpoint' ends up meaning. I can let you remove it; I can maybe let you put a flamer there, if I'm feeling generous. But I can't really let you put a genuinely useful weapon in that spot.
The machine guns comment was trolling, I admit. You will be able to remove them. I'm just thinking about how to preserve the 'flavor' of a stock 3025 'Mech while still allowing customization. That means that you don't get to replace a machine gun with an AC/10, for example.
You probably can't fit an AC/20 into wherever an AC/2 was mounted. The location has an overall size limit.
MGs and flamers I'm currently thinking of as 'antipersonnel weapons' with their own 'flavor' of hardpoint, distinct from energy, ballistic and missile.
Hardpoints. And size restrictions, but those are easy. Mostly hardpoints. So that you need to put missiles in your Catapult C1's arms. Other variants of the chassis, where they exist, are distinct chassis from the default. So if you find a K2, it's a different chassis than the C1, and they're not interchangeable.
In the single-player campaign, you will need to spend time, money, and have MechTechs and a MechBay available in order to customize a 'Mech.
(to retain 'Mech flavor): avoidance of degenerate gameplay. 10xMG is degenerate, but so is the typical alpha-striking laser boat. I don't necessarily want to enforce the flavor at the expense of personal creativity and strategy, but I want to keep it within a reasonable 'feels-like-BT-lore' space.